1. Field of the Invention
The present invention relates to a semiconductor device such as a DRAM (dynamic random access memory) element including a 6F2 memory cell whose size is defined by a numerical value of a design rule F. More specifically, the present invention relates to a semiconductor device including a memory cell part having a supporting part for preventing lower electrodes of capacitors of a memory cell part from collapsing and coming into contact with each other.
2. Description of the Related Art
Developments in miniaturization of semiconductor devices have led to smaller areas of memory cells making up a DRAM. Consequently, it is generally practiced that capacitors making up a memory cell be three-dimensionally formed in order to secure sufficient capacitance at the capacitors. Specifically, by providing a lower electrode of a capacitor with a cylindrical shape and using a sidewall of the lower electrode as a capacitor, surface area can be expanded and capacitance can be increased.
The reduction in memory cell area has lead to a reduction in the area of the bottom of lower electrodes of capacitors. As a result, a phenomenon in which a lower electrode falls over and short-circuits an adjacent lower electrode (collapse) is more likely to occur in a manufacturing process for exposing an outer wall of a cylindrical capacitor. In order to prevent such collapsing of electrodes, techniques are proposed in which a support film for providing support is disposed between lower electrodes (Japanese Patent Laid-Open No. 2003-297952, Japanese Patent Laid-Open No. 2003-142605).
As described in Japanese Patent Laid-Open No. 2003-297952 and Japanese Patent Laid-Open No. 2003-142605, collapsing of a lower electrode can be prevented by horizontally and vertically connecting the lower electrodes of adjacent capacitors with a support film.
Meanwhile, in response to demands for higher integration of DRAMs, a technique is known for reducing memory cell size by arranging a memory cell in a layout referred to as a 6F2 layout (Japanese Patent Laid-Open No. 2007-287794). As described in Japanese Patent Laid-Open No. 2007-287794, a 6F2 memory cell structure enables an increase in capacitance by arranging capacitor elements in a closest-packing arrangement in a given area. In addition, even with a 6F2 memory cell, by arranging a support film that prevents collapsing of the lower electrodes of capacitor elements, capacitance can be further increased by using the sidewalls of cylindrical lower electrodes.
However, the present inventor has discovered that providing capacitor lower electrodes arranged in a closest-packing arrangement with a support film for preventing collapsing has the following problems.
FIG. 16 illustrates a schematic plan view of a case where lower electrodes of adjacent capacitors are connected and supported by linear support films extending in vertical (Y) and horizontal (X) directions as described in Japanese Patent Laid-Open No. 2003-142605. Reference numeral 101 denotes positions where lower electrodes of capacitors are to be placed. In a 6F2 memory cell arrangement, six lower electrodes 101 are arranged around one lower electrode 101 to form hexagon 102. Respective lower electrodes 101 are supported by support film 103 linearly extending in an X-direction and support film 104 linearly extending in a Y-direction. Such a support film pattern can be formed by depositing silicon nitride (Si3N4) as a support film and patterning the silicon nitride using a photomask provided with openings 105 at places other than those where support films 103 and 104 are to be formed. With a support film arrangement such as that illustrated in FIG. 16, the small sizes of openings 105 cause the following problems.
Firstly, in a wet etching process for exposing the side surfaces of the lower electrodes of capacitors, the penetration velocity of a chemical (hydrofluoric acid) to an interlayer insulating film under the support film is reduced. Consequently, the support film itself ends up being exposed to the chemical over a longer period of time. Therefore, in wet etching using hydrofluoric acid, since etching of silicon nitride that makes up the support film also proceeds gradually, the extended exposure to the chemical problematically damages the support film and impairs the function of supporting the lower electrodes.
Secondly, with the generation of miniaturized elements based on a design rule of 70 nm or less, there is a problem in that it is difficult to accurately form a pattern of openings 105 with minute sizes even with a state-of-the-art exposure apparatus.
In consideration of the above, the present inventor performed an evaluation on a support film described in Japanese Patent Laid-Open No. 2003-297952 and linearly extending only in one horizontal or vertical direction but modified so as to accommodate a 6F2 memory cell. FIG. 17 illustrates a schematic plan view of a support film arrangement used for the evaluation. Lower electrodes 101 are arranged so as to form hexagon 102. Reference numeral 110 denotes a support film arranged so as to linearly extend in an X-direction. A linear opening pattern with approximately the same width as the support film is provided between adjacent support films 110.
With the support film arrangement illustrated in FIG. 17, the sufficiently-large area of opening portions provided in the support film solved both the first and second problems described with respect to the arrangement illustrated in FIG. 16. However, when attempting to arrange a large number of lower electrodes with high aspect ratios in the arrangement method illustrated in FIG. 17, an extended length in the X-direction of the support film fixed at both ends thereof caused new problems including curving (displacement) of the support film in the Y-direction as seen in the diagram and an occurrence of partial cracks 111 in the support film itself. Therefore, collapsing of the lower electrodes could not be prevented.
In consideration of the problems described above, the present invention provides a semiconductor device having a readily-manufactured support film with sufficient supporting strength to prevent collapsing of lower electrodes of capacitors.